1. Field of the Invention
The invention relates to a magnetization device for permeation of a multiphase fluid flowing through a measurement tube of a nuclear magnetic flow meter with a magnetic field which is homogenous at least in one plane, with a plurality of permanent magnets for generation of a magnetic field and with a carrier, the carrier having at least one magnet receiver, each of the magnet receivers accommodating at least one of the permanent magnets, the shape of the magnet receivers and of the permanent magnets allowing movement of the permanent magnets only in one direction in the magnet receivers and the permanent magnets held by the magnet receivers being arranged by the magnet receivers with reference to the magnetic field.
2. Description of Related Art
A nuclear magnetic flow meter determines the flow of the individual phases of a multiphase fluid, the flow velocities of the individual phases and the relative proportions of the individual phases in the multiphase fluid in the measurement tube by measuring and evaluating the voltage induced by the nuclear magnetic resonance of the multiphase fluid into a suitable sensor. The measurement principle of the nuclear magnetic resonance is based on the property of atomic nuclei with a free magnetic moment to process to the nuclear spin in the presence of a magnetic field. The precession of the vector representing the magnetic moment of the atomic nucleus takes place around the vector representing the magnetic field in place of the of atomic nucleus, the precession inducing a voltage into the sensor. The frequency of precession is called the Larmor frequency ωL and is computed according to ωL=γ·B, γ being the gyromagnetic ratio and B being the amount of the magnetic field strength. The gyromagnetic ratio γ is maximum for hydrogen nuclei, for which reason especially fluids with hydrogen nuclei are suited for nuclear magnetic flow meters.
A multiphase fluid composed essentially of crude oil, natural gas and salt water is delivered from an oil source. So-called test separators branch off a small part of the delivered fluid, separate the individual phases of the fluid from one another and determine the proportions of the individual phases in the fluid. Test separators are expensive, cannot be installed under the sea and do not allow real-time measurements. In particular, test separators are, however, unable to reliably measure crude oil proportions smaller than 5%. Since the crude oil proportion of each source drops continuously and the crude oil proportion of a plurality of sources is already less than 5%, it is currently impossible to exploit these sources.
Both crude oil and also natural gas and salt water contain hydrogen nuclei, for which, as already mentioned, the gyromagnetic ratio γ is maximum. Nuclear magnetic flow meters are therefore suited especially for use on oil sources, also undersea directly on the source on the sea bed, but are not limited to this application. Other applications arise, for example, in the petrochemical or chemical industry. Branching off of the fluid is not necessary, and the entire fluid is measured in real time. Compared to test separators, nuclear magnetic flow meters are more economical and require less maintenance and can also especially reliably measure crude oil proportions less than 5% in the fluid, as a result of which the further exploitation of a host of oil sources becomes possible for the first time.
It is immediately apparent from the equation for computing the Larmor frequency ωL that the Larmor frequency ωL is proportional to the magnetic field strength B, and thus, the magnetic field strength B also acts directly on the voltage induced into the sensor. Heterogeneities in the magnetic field therefore reduce the measurement quality of nuclear magnetic flow meters, for which reason the task of the magnetization device is the permeation of the fluid with a magnetic field which is ordinarily homogeneous within the measurement tube. The required measurement accuracy determines the necessary homogeneity of the magnetic field. Often measurement methods are used which use a known gradient in the magnetic field so that the magnetic field is constant only in one plane.
U.S. Pat. No. 7,872,474 B2 discloses a magnetic resonance based apparatus and method to analyze and measure bi-directional flow that utilizes a stack of disks, formed of a plurality of bar magnets, which forms a hollow cylindrical permanent magnet, the magnetic field being homogeneous in the cylindrical interior of the magnet. The magnets of each disk are held between rings of non-magnetic material and fixed by the screws that are also made of non-magnetic material the discs of magnets piled up and held by non-magnetic screws.
In each individual disk of magnets forms a Halbach array. The important feature of a Halbach array is that the magnetic field forms largely on one side of the Halbach array, here, in the interior of the cylindrical magnet, and on the other side, only a very weak magnetic field forms, here, in the external space of the cylindrical magnet Since a strong magnetic field is required for high voltages induced into the sensor by the precession of the hydrogen atoms contained in the fluid, correspondingly strong bar magnets are used. Due to the plurality of bar magnets which are arranged tightly in each of the magnet disks, the introduction of the bar magnets into the magnet receivers is associated with a high expenditure of force. Moreover, the resulting magnetic field is initially not homogenous enough, for which reason the magnetic field must be made homogeneous by manipulation on each of the bar magnets. This process is called shimming. The introduction and shimming of the numerous bar magnets mean a considerable production and time expenditure, which is accompanied by the corresponding costs.